Olivine structure lithium metal phosphates LiMPO4 are cathode active materials in lithium ion batteries, with advantages including low-cost, environmental friendliness, high abundance, stable chemical properties, and excellent safety. One lithium metal phosphate is lithium iron phosphate (LiFePO4), which has a theoretical capacity of 170 mAh/g and superior cycling capability. However, LiFePO4 has a voltage plateau of 3.4 V, which is a severe restriction of energy density of the lithium ion battery. LiMnPO4, LiCoPO4, and LiNiPO4 have better energy density, but have relatively low electronic conductivities and lithium ion diffusion rates restricting the applications thereof. A solution for this problem is doping the lithium metal phosphates to form LiMxN1-xPO4 (e.g., LiMnxFe1-xPO4 or LiMnxMg1-xPO4).
Solvothermal synthesis and solid phase synthesis are two methods for synthesizing the doped lithium metal phosphates LiMxN1-xPO4. The solvothermal synthesis includes dissolving reactants in an organic solvent and solvothermal heating the solution in an autoclave to obtain the doped lithium metal phosphate. The solid phase synthesis includes ball milling a mixture of the reactants with a solvent, and calcining the milled mixture in an inert gas at a relatively high temperature to obtain the doped lithium metal phosphate.